Cardiac arrest, 3 phase:
electrical abnormality phase. Here where Defib came along. Shock! Shock! Shock! If shockable rhythm.
5-20 min (usually time arrive at hospital):
circulatory abnormality. Here where CPR and epinephrine work. Vasoconstric (alpha effect) and let the blood return to essential organ (heart, lungs and brain).
20 min plus:
metabolic state, CPR and ventilation. Correction of metabolic error. More epinephrine will stimulate the beta effect and cause increase in myocardial oxygen demand(MOD). Hurting the heart more.
Anyway, the epinephrine use in cardiac arrest still base on limited data.